a. Field of the Invention
The present invention relates to field monitoring of riser systems used in the oil and gas industry. However, this method can be used for any type of similar structure such as cables, umbilicals, mooring lines and pipeline spans. In particular, this invention relates to the use of motions measured at a single location, in lieu of strain sensors, for curvature and fatigue damage monitoring under excitation from any source.
b. Description of the Related Art
Risers are used in the oil and gas industry as the pipe conduit between the ocean seabed and the surface. Risers and similar structures such as cables, umbilicals and mooring lines are exposed to the environment and are susceptible to excitation from various sources which can cause fatigue accumulation in the structures. Fatigue monitoring can be used to determine the riser fatigue accumulation, confirm riser integrity and assist in operational decisions.
Fatigue monitoring can be achieved through strain and curvature measurement, motion measurements, or a combination of these. The benefit of strain or curvature measurement is that it is a direct measurement. However, depending on the application, subsea strain measurement may not be feasible. In addition, subsea strain measurement may not have the required track record, reliability and cost effectiveness of motion measurement. Hence, it is preferable to have a means of determining curvature from motion measurements.
A number of methods currently exist to determine fatigue damage from motion measurements. Typically these take the form of reconstruction methods using mode shapes and/or mode superposition. As an example US Patent 2012/0303293 A1 presents a method based on the use of time synchronous measurements and mode superposition. Another approach is to use finite element analysis (FEA) to determine acceleration to stress transfer functions.
The objective of the present invention is to provide an improved monitoring means allowing continuous measurement of riser fatigue from accelerations without the necessity for strain or curvature measurement devices, time synchronous measurements, theoretical mode shapes and/or finite element analysis (FEA).